However the the Arduino team, wanting a full open source platform, probably PICed (pun!) AVR because of the open source AVR/GCC C/C++ compiler that they could build on top of.
That is probably the main reason. I don't know of any open-source PIC compilers (I would love to see an open source PIC Basic compiler).
I started out using the Arduino when I moved my linux workstation to a 64-bit distro (Ubuntu); I had been using an old 32-bit install of SuSE up until then. I was playing with a Basic Stamp 2 (based on a PIC) module I had (from Parallax) using their free tokenizer library and some glue scripting I created for my editor (so I could easily compile and upload to the Stamp). It worked pretty well, and I was beginning to do some serial comms stuff with the Stamp when I migrated.
The tokenizer library Parallax supplied no longer worked - it was written for 32-bit only, and there was only a binary supplied that was statically linked (!) to other libraries on the system, so there was no way to work around it (like using IA32 libs - I tried). I contacted Parallax, and they told me that they had no further contact with the programmer of the tokenizer library and they didn't have the source code, either. I was perfectly willing to compile it myself for 64-bit had they had the source code...
I did some more searching, and found Processing and the Arduino, and was instantly hooked. A quick order thru SparkFun and a week later I had the blink sketch running.
I have been burned too many times by closed source; the only thing I run currently now that is closed-source is the NVidia OpenGL drivers - I'll probably be burned by them someday, too (but what choice do I have for high-speed 3D graphics under linux?).